Where is the operator panel in Blender 2.8?
It can be found in the 3d Viewport:

As well as in the Edit menu where it is called Adjust Last Operation now, where you can also right click it and choose Assign Shortcut if you wanted to be able to call it with a keyboard shortcut like it was with F6 in previous versions:

Obviously, it does not need to be F6 and could be anything that is convenient to you.